1. Field of the Invention
This invention relates to a non-aqueous electrolyte cylindrical primary battery having high-rate discharge characteristics comprising a strip-like positive plate including manganese dioxide or graphite fluoride as the cathodic active material and a strip-like negative plate of lithium or other such light metal as the anodic active material spirally wound with a separator in between to form an electrode assembly, and an organic electrolyte.
2. Description of the Prior Art
Non-aqueous electrolyte batteries using a light metal such as lithium as the anodic active material have various advantages such as superior low temperature performance, wide working temperature range, less self-discharge, but have been inferior in high-rate discharge characteristics. The non-aqueous electrolyte cylindrical primary battery with a spirally wound electrode plate structure (group) was developed for long-life use in automatic-focusing still cameras so as to function properly under high-rate discharging by the motor driven focusing, film advancing and rewinding or electronic flash lighting.
The electrodes positive and negative, of this type of cell are prepared usually as a long thin band. The positive electrode is fabricated by filling a core material of expanded or punched metal sheet with a high density positive mixture consisting of cathodic active material, conductive material, binder etc. In recent years, the negative electrode for a primary battery with the negative electrode of lithium, a simple type consisting of high conductivity lithium foil with a lead plate fixed at a required location has been used instead of a lithium foil pressed into the entire surface of a skeleton of expanded metal provided with a lead plate as was first developed. Such a structure, results in increased quantity of cathodic active material and low cost.
The positive and negative electrodes described above are wound with a separator of microporous polyolefin (polypropylene and/or polyethylene) membrane to form a spirally wound electrode plate group. This structure results in an increased area of the positive and negative electrodes and the current density being substantially lowered, resulting in high reaction efficiency of both the positive and negative electrodes during high-rate discharging.
In recent years, the use of such a battery has been expanded, not only for AF cameras, but also for various other devices such as town gas, water supply, or electric power supply meters for electronic devices such as communication equipment, measuring equipment, or for semiconductor back up memories. Accordingly, the pattern of the discharge of the battery has become varied, e.g. continuous discharging in a weak current of a microampere order, low-rate discharging with occasional high-rate discharges as necessary for the operation of electromagnetic valves.
Referring to FIG. 1, there is shown a top plain view of an electrode plate structure (group) spirally wound as previously described, the group comprising a negative electrode 1, negative electrode lead plate 1a, positive electrode plate 2, positive electrode lead plate 2a, separator 3, with a space 4 in the center. As a result of investigation, the negative electrode lead plate 1a is fixed at a portion of the positive electrode plate 2 a short distance inside from the outermost end 2b of positive electrode plate 2, in order to obtain higher reaction efficiency of anodic active material in the high-rate discharge mode. Although not shown in the drawing, the negative electrode lead plate 1a, positive electrode lead plate 2a, and the outermost end 2b of the positive electrode 2 are provided with a cover of insulating adhesive tape, to prevent internal short circuiting due to irregular surface projections.
In general, the battery has higher reaction efficiency or utilization of active materials of both the positive and negative electrodes and a high discharge capacity, when used in low-rate as opposed to high-rate discharging. Conventional non-aqueous electrolyte cylindrical battery with superior high-rate-discharge characteristics, at the low-rate discharge, did not always indicate higher capacity but sometimes did show inconsistency. Observing many batteries disassembled after low-rate discharging, it was learned that the negative electrode plates of lithium were not uniformly reacted by discharge, some parts were completely dissolved and extinguished, while other parts partially reacted to become thinner, or parts not reacted remained as they were, leaving the lithium foil in pieces, as shown in FIG. 5. In FIG. 5, the outline 51 of the original negative electrode plate proper of lithium is seen, with the remaining parts shown outlined by solid lines and the parts which have vanished by no cross-hatching. The parts having oblique cross-hatching are those which have remained as lithium metal after low-rate discharge. The portion of the negative electrode lead plate 51a is fixed to the electrode plate 51 and is covered with an insulating adhesive tape 51b thus preventing contact with the electrolyte, leaving the lithium foil not dissolved or consumed. Neither the outermost end 51c of the negative electrode plate nor the part 52d and 52c facing the lead-plate-fixing part of the positive electrode with the adhesive tape in between or the portion of the tape 2b (as shown in FIG. 1) was dissolved or consumed. The portion 51d forming the outermost part of the electrode group having one surface facing the positive electrode was dissolved at the inner side which faces the positive electrode but the lithium foil on the opposite side has been left unchanged. The part 51f is considered to have remained in the metal state until the end of discharge, if the current density was low and the discharge reaction was delayed. What is remarkable on all of the negative electrode plates of the conventional batteries is that the part of the lithium foil between the mounting point of the negative lead plate and the part facing the mounting point of the positive lead plate with separators in between are by and large dissolved causing the foil to divide. This is attributed to the high current density at the lead-fixing points of the positive or negative electrode plates. As shown in FIG. 5, the lithium foil of the negative electrode after the discharge has its peripheral edges dissolved away with metal pieces remaining, which were separated from the negative electrode lead plate 51a naturally terminating electrical connection therewith and accordingly failing to contribute to the reaction at its last phase.
Thus, the unexpected low capacity of the conventional battery under low-rate discharge is considered due to incomplete use of the lithium metal when used as a negative electrode. Such phenomena is more prominent in the case of the negative electrode capacity being nearly equal to or less than the positive electrode capacity. Thus, to improve the low-rate discharge performance of the non-aqueous electrolyte cylindrical battery having a negative electrode plate of lithium foil provided with a lead plate fixing location for high-rate discharge use, it is important to have the lithium foil not separate from the lead plate to maintain the electrical connection until the end of the discharge.
In order to solve the problem of the conventional batteries proposed solutions are disclosed in Japanese patent applications Laid-open No. Sho 61-281465 and Sho 61-281465, for example. The former application discloses provisions of a groove on the positive electrode of strip disposed along the winding direction of the electrode, and the latter application teaches putting an insulating layer on the negative electrode of lithium by applying, in the winding direction, an insulating adhesive tape or a covering of an insulating paint. However, the first disclosure could not be applied to the positive electrode plate having a core material such as an expanded metal or a net which are cubic and have good adhesion to the positive electrode mixture, due to the difficulty of forming a groove in the electrode. Also, a positive electrode plate formed from a perforated core sheet such as a punched metal with a groove formed thereon, has a shortcoming in that the mixture filling the core partially dropped when the electrode was wound up, often resulting in internal short circuiting of the battery.
In regard to the second invention, it is necessary, to form an insulating layer on the lithium foil by applying the tape in dry, inert, low-temperature atmosphere, or, to dry after the application of the paint, because of reaction of the foil with water, etc. both resulting in processing difficulties.